Osteoporosis is a major public health problem. Optimum development of the skeleton during youth is crucial in prevention of osteoporosis, a condition in which fractures occur late in life because of reduced bone mass and bone quality. If skeletal development can be maximized during growth, young people will begin adulthood with optimal bone health and be less likely to develop osteoporosis in later years. Yet bone health of youth has not been adequately addressed. In this application, we propose to extend our 4-year intervention study of the effects of a high calcium diet on bone quality in pubertal girls. The aim of this experimental study is to test the effect of increasing dietary calcium intake to 1500 mg/day on increases in bone quality in adolescent females. The hypothesis to be tested is: adolescent females who consume a high calcium diet will have a greater increase in bone quality, as measured by densitometry and ultrasound, than adolescent females who consume their usual dietary calcium intake. The study uses an experimental design in which 58 participants (26 treatment and 32 control) will be followed for a total of 9.5 years. Participants are a convenience sample who were age 9 when enrolled. Girls in the treatment group will continue to be given high calcium foods to supply at least 1500 mg/day, while girls in the control group will asked to continue consuming their usual diet. Measurements to be made at baseline, every six months, and at the end of study include: bone quality determined with densitometry and ultrasound, height, weight, Tanner stage, and medical and social history. Usual physical activity and dietary intake will be determined every three months. Funding of this project in the next cycle would enable us to follow these young females continuously from age 9 to 18.5. To our knowledge, this would be the first intervention to evaluate the effects of optimal calcium intake on bone health throughout the period of highest bone mass and bone quality accrual. It is critical that we take advantage of this opportunity. This enthusiastic group of adolescent participants provides us with confidence that we can successfully complete this ambitious project. Evidence of a positive effect of increased calcium intake from food sources on bone quality in adolescent females would provide strong support for educational efforts targeted at adolescent females, parents and care providers. Positive findings would also provide the impetus for calcium fortification of more foods that appeal to young females. Lack of a calcium effect would provide support for further research into other methods or concomitant methods of maximizing skeletal development.